Since this thread has been revived, I'll chime in. Blanton's was, indeed, the first single barrel bourbon but, as someone observed, the concept of "single barrel" as something unique has to post-date the practice of dumping multiple barrels and mingling their contents together, which began when bottling itself became common early in the 20th century. Before that, whiskey was sold by the barrel, from which smaller units of sale were then drawn, so everything was "single barrel" then.

Another caveat is that Blanton's got the idea from single cask bottlings of single malt scotch, which came along earlier, so it wasn't a particularly original idea. That, to me, makes the fact that Maker's Mark may have toyed with it not all that significant. It's a pretty obvious notion. The problem with it is you need to build a special line for it, so it's a little more than just a marketing concept. There actually is some cost and commitment involved.

But Blanton's was, indeed, the first American whiskey to adopt the single cask concept.